In order to calculate an average over many projects, you need a single record that all projects are linked to. So…
Table 1: Sales team (here you literally only have one record called “Sales Team”. You could have multiple teams of course.
Table 2: Projects (this is where the form enters your data) and every project / record is linked to the sales team.
In table 1, you can now create a Rollup field and calculate the average of all values. In another Rollup field, you build the sum of all clicks. In a formula field, you subtract that sum from your 500.000 total clicks.